In order to better recognize the benefits and the weaknesses of scintillator-based X-ray detectors one must understand some of the aspects of the interactions between X-rays and atoms.
The most favored interaction-mechanism between the X-ray photon and the scintillator is photoelectric scattering where X-ray absorption is immediately followed by emission of a fluorescence-photon. The attenuation-coefficient for fluorescence-photons is significantly lower than the attenuation-coefficient for photons of energy slightly above the K-shell energy, such that the locations where the X-ray photon and the associated fluorescence-photon are stopped may be hundreds of microns apart.
A large body of information on the photoelectric scattering and competing channels can be found in the paper by Walter Bambinek et. al published in the Review of Modern Physics, vol. 44, N. 4, October 1972.
Following the interaction between an X-ray photon and scintillator atoms a significant fraction of the X-ray photon-energy is converted into visible-photons. A photodiode is usually attached to one of the scintillator-walls and generates an electrical-signal when struck by visible-photons. The signal is further processed by electronic-circuitry to obtain a pulse whose amplitude is a measure of the energy deposited in the specific scintillator-volume by the detected X-ray photons.
Most X-ray detectors are designed to achieve: efficient X-ray absorption, high geometric fill-factor and high position-resolution. To reach better and better position-resolution one chooses to use smaller scintillator-elements (pixels). Modern detectors may display pixels of width as small as 0.05 mm.
The fraction of the X-ray energy converted into visible-photons close to the location of the first interaction is directed towards the attached photodiode by thin light-reflecting septa or by the columnar structure of the scintillator. The fluorescence-photon energy will often be deposited in another pixel (the signal-sharing effect) or may be lost if the fluorescence-photon escapes the scintillator.
The immediate damage is twofold: first the electrical-pulse associated with X-ray detection is not an accurate measure of the photon-energy and, second, an electrical-signal is generated in a neighboring pixel as the fluorescence-photon is stopped there.
To demonstrate the negative impact of signal-sharing on image quality, consider the use of a CsI (T1) detector in medical-imaging applications. The energy distribution of X-ray photons leaving the human-body, under typical radiographic-imaging conditions, peaks close to 60 keV. Given that each fluorescence-photon in CsI (T1) carries about 30 keV, and that about 50% of the fluorescence-photons are stopped in a typical 0.2 mm pixel, roughly half of the energy associated with detection of an X-ray photon is deposited outside the pixel where the impinging photon is stopped. The result is a significant reduction in image-contrast.
Some detection-systems, like the Medipix-3 system developed at CERN, use sophisticated electronics-circuitry to identify all signals generated by a single X-ray photon, within a given time-interval, in adjacent pixels. The system then estimates the original interaction-location and the X-ray photon energy. The complexity of this approach prevents its implementation in commercial equipment.